The present invention relates to a method of acquiring seismic data and particularly to a method of acquiring seismic data in an area having periodic acoustic interference.
Seismic data is typically acquired by conducting a seismic survey over a particular subsurface area of interest, generally in connection with hydrocarbon (oil and/or natural gas) exploration and/or production activities. In order to perform a 3D marine seismic survey, for instance, a plurality of seismic streamers, each typically several thousand meters long and containing arrays of acoustic receivers, known as hydrophones, and associated electronic equipment distributed along its length, are towed at about 5 knots behind a seismic survey vessel, which also tows one or more acoustic sources, typically air guns, that are capable of producing seismic signals. The seismic signals (or xe2x80x9cshotsxe2x80x9d) produced by the acoustic sources are directed down through the water into the earth beneath, where they are reflected by the various geologic strata. The reflected signals are received by the hydrophones in the streamers, digitized and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed. The processed seismic data allows companies to make better decisions regarding where to drill for hydrocarbon deposits and how best to manage the production of hydrocarbons from subsurface reservoirs.
Data quality problems can arise when seismic data is acquired in an area having periodic acoustic interference. This can occur, for instance, when two seismic survey vessels are carrying out marine seismic surveys in the same general area, say within 50 to 60 kilometers of each other. Acoustic interference occurs when the seismic signals produced by one of the vessels can be detected in the seismic data being gathered by the other vessel. At times, the seismic signals from each of the vessels will interfere with the seismic data being collected by the other vessel. Often, however, due to geology, changes in water depth, or the willingness on some contractors part to collect data that is impaired, only one of the vessels will be affected. The interference will typically be of short duration, perhaps one to two seconds, and the interference will be relatively evenly spaced in time, because the cycle time of the interference will be related to the speed and the desired shot point interval of the seismic survey being conducted by the other vessel.
This type of interference is becoming more and more common as seismic data is being acquired not only in connection with hydrocarbon exploration activities, but also in connection with hydrocarbon production activities. Ongoing seismic monitoring of hydrocarbon reservoirs and integrating this seismic data with wireline log and petrophysical data allows for more effective management of reservoir development and production activities. This increased need to acquire seismic data from hydrocarbon production areas, which are often clustered in relatively limited geographic regions, will tend to increase the frequency at which seismic signals from one seismic survey will interfere with the seismic data being collected by another seismic survey in the same general area.
One method for eliminating this type of interference is for the seismic contractors operating in a particular area to enter into cooperative agreements, referred to in the industry as time share agreements. In these agreements, the seismic data acquisition crews agree to conduct their seismic surveys in a particular area at different times to avoid collecting data showing interference from the other seismic survey crew. There has been a tendency recently, however, for time share agreements to be resisted by some seismic contractors. This has been especially common when dealing with certain Ocean Bottom Cable crews, who claim that their equipment is less susceptible to interference and they should have a 3 to 1 time share agreement because they have more boats on their crew. Both of these claims are highly questionable. A more significant problem with time share agreements, however, is that they are inherently inefficient. By allowing only a single vessel to acquire seismic data in a particular area at any given time, the quantity of seismic data available from that area will be significantly less than the quantity of seismic data that could be acquired by two or more vessels operating in the same area (if each of the vessels could acquire seismic data that was not contaminated by seismic signals from the other vessels).
Another method for attempting to eliminate this type of interference is described in U.S. Pat. No. 5,424,999 (Manin). In the method described in this patent, the spacetime coordinates of the seismic vessels are recorded and the propagation velocity of seismic waves through the off-shore environment is used to calculate the travel time of the noise being produced by the first vessel. This type of method suffers from two primary drawbacks. First, it requires the seismic survey vessels to continuously exchange information regarding their positions and shot timing. In many cases, the vessels will be operated by different operating companies and they will not have an incentive to assist their competitors by providing this type of information. Second, and even more significantly, the non-homogeneous nature of acoustic transmission in a marine environment makes it extremely difficult for this type of method to accurately estimate the travel time of the noise signal. The velocity of sound waves in sea water varies based factors such temperature and pressure. The acoustic velocity of sea water is typically minimized at approximately 1000 feet and this zone of minimum acoustic velocity produces a channeling or lensing effect that causes the acoustic wavefronts to travel in ways that are not predicted by simple spherical spreading models. This non-homogeneous acoustic transmission characteristic makes it virtually impossible to accurately calculate the acoustic pulse travel time between two relatively widely spaced apart marine locations, particularly when these locations are constantly moving with respect to each other.
It should also be noted that some types of non-seismic activities, such as certain types of construction and drilling activities, can also produce periodic acoustic noise and these types of acoustic signals can also interfere with the acquisition of seismic data in the immediate vicinity of these activities.
It is an object of the present invention to alleviate these problems and to improve the quality of seismic data acquired in an area having periodic acoustic interference.
An advantage of the present method is that it does not require seismic survey vessels (particularly vessels operated by different operating companies) to continuously exchange information regarding their positions and shot timing. A further advantage of the present method is that it does not require the travel time of the noise signal through the non-homogeneous marine environment to be calculated.
According to the present invention, there is provided a method of acquiring seismic data using an acoustic source capable of producing a seismic signal and an acoustic receiver in an area having periodic acoustic interference, the method comprising the steps of determining a time window in which the periodic acoustic interference at the acoustic receiver is substantially attenuated and adjusting the acoustic source""s discharge time so a desired portion of a seismic signal produced by the acoustic source arrives at the acoustic receiver within a subsequent time window.
The inventive method may be implemented during a marine seismic survey by performing the following steps: detecting when the periodic acoustic interference at the acoustic receiver is of sufficient magnitude to require adjusting the acoustic source""s discharge time; determining the periodic acoustic interference""s cycle time; switching the seismic survey vessel""s control system from position mode shooting to time mode shooting; adjusting the seismic survey vessel""s speed to provide for the proper shot point spacing; determining whether the periodic acoustic interference is arriving at the same time as a desired portion of the seismic signal; if the periodic acoustic interference is arriving at the same time as a desired portion of the seismic signal, then determining what time shift would allow the desired portion of the seismic signal to arrive at the acoustic receiver during a subsequent time window in which the periodic acoustic interference is substantially attenuated and adjusting the acoustic source""s discharge time by this time shift; monitoring the periodic acoustic interference and the desired portion of the seismic signal; further adjusting the acoustic source discharge time when required to allow the desired portion of the seismic signal to continue to arrive at the acoustic receiver during a time window in which the periodic acoustic interference is substantially attenuated and further adjusting the vessel""s speed when the acoustic source discharge time is further adjusted.
If the duration of the time window where the periodic acoustic interference is substantially attenuated is sufficiently large and the desired portions of the seismic signals arriving at the acoustic receiver are sufficiently short, the desired portions of more than one seismic signal may be received within each successive time window using the inventive method. Similarly, if the minimum cycle time of the acoustic source or some other aspect of the seismic data acquisition system does not allow a desired portion of a seismic signal to be received within each successive time window, the desired portions of the seismic signals may be received at a cycle time that is a multiple of the periodic acoustic interference""s cycle time, i.e. the desired portions of the seismic signals may be received once every N time windows, N being greater than or equal to 2.
This method may be used, for instance, to acquire seismic data using a seismic source and a plurality of seismic receivers which are towed behind a seismic survey vessel, in the presence of interfering signals from another acoustic source being operated by another vessel in the same general vicinity.